The invention relates to a microstrip-line circuit disposed on one face of an insulating substrate, the other face of which carries a ground plane, the strip line being used as a resonant element whose tuning frequency may be modified. A mechanical adjustment device is provided which has at least one moving part which includes at least one electrically conductive part.
A strip line of this kind is used chiefly as an oscillator-tuning element for frequencies an the order of a gigahertz or over, in systems for the reception of satellite television signals. In these systems there is an element located downstream from the aerial lead-in cable, in which is led a signal whose frequency is between 950 MHz and 1.75 GHz. This element is provided with input circuits tuned to a high frequency, followed by a frequency-mixer stage comprising a local oscillator, and delivering an intermediate frequency signal.
When this element is being tuned, the tuning frequency of the input circuits and that of the local oscillator are varied simultaneously. During this operation it is difficult to obtain a frequency variation which is identical for the input circuit and for the local oscillator.
The known solutions used for tuned circuits formed on the basis of variable capacitors and/or wire-wound inductors are not applicable in the present case, for the frequencies in question, since a resonant element consisting of a microstrip line is used, in association with variable-capacity diodes. The manufacturing spreads of these diodes are such that the frequency variation obtained for a given variation of the voltage across the diode is not sufficiently reproducible to make it possible to ensure that the frequency of the oscillator and that of the high-frequency tuning circuit follow each other with a constant difference when the frequency is varied. It is therefore necessary to provide an additional fine adjustment.
A solution for tuning a resonant strip line is described, for example, in the publication, "IEEE Transactions on Microwave Theory and Techniques", Vol. MTT 17, no. 12 (December 1969), pp. 1069-1071. It consists of varying the tuning frequency by altering the position of short-circuits formed by screws passing through the substrate towards the ground and utilising adjustable capacitive elements consisting of gold-plated steel plates which can be moved along the line and are held in position by magnets placed under the substrate. Changing the position of short-circuits from one hole to another produces a variation in steps without intermediate settings, which makes it possible to obtain fine adjustment, and the movable plates held in position by magnets form a possible solution for experimental use as described in the document referred to above, but would be difficult to put into operation in an equipment subject to shocks and vibrations, which might perhaps displace the plates. A mechanical system permitting the longitudinal movement of the plates along the line while ensuring their electrical contact with the line and their solid attachment would be difficult and costly to achieve. Additionally, it is not just a matter of adjusting a strip line for a given frequency but of obtaining a particular frequency-variation curve when a tuning element is varied, particularly the control voltage of a varicap diode located at the end of the line. It is not obvious that the movement of capacitances formed by plates according to the document referred to can resolve this problem.